ski

It's a "Robbing Peter To Pay Paul" situation with that resistor. When the IAT signal to the PCM is artificially lowered, the PCM not only increases the timing but it also richens the fuel mixture. This means the former is increasing power, while the latter is reducing it. Plus, the former will increase power only if the engine burns fuel that has an octane rating which is higher than what the tune is designed for. Otherwise, the knock sensors will pull timing, which means that the engine will not only lose power due to that, but it will also lose power due to the richer fuel mixture.
The proof is still in the numbers. Ask the supplier for dyno charts, and a means to contact the owners whose cars were dynoed. If he has nothing in that regard, then chalk it up as nothing more than snake oil.